Telephone connections that use long distance four-wire circuits coupled to local two-wire circuits by means of hybrid couplers, give rise to echo phenomena due to imperfect balancing of the hybrid couplers. These echo phenomena are a great hindrance to a speaker once the echo delay exceeds 45 milliseconds, and unless the echos are attenuated speech becomes incomprehensible. To do this, echo suppressor devices are used which, when only one party is talking, apply a high degree of attenuation in the talking party's reception channel, and when both parties are talking apply a medium degree of attenuation to both directions of transmission. These devices may act by means of controlled attenuators inserted in the path of the speech signals, in which case they are actually called echo suppressors. However, they may also act by means of sensitivity controls on speech detectors in devices for concentrating telephone signals by assigning a physical speech channel to a party only when that party is talking, such devices are known under the initials TASI (or CELTIC in French) and they are used to increase the number of conversations which can be simultaneously carried over some connections of limited capacity.
This method of attenuating echos as commonly used for speech signals is not suitable for signalling and data signals which are very often simultaneously transmitted in both transmission directions, which means that the echo suppressor devices must either be inhibited or removed from circuit during data transmission or signalling.
One known way of detecting signalling and data signals on a telephone channel is based on an analysis of the frequencies in the telephone channel, and interpreting as speech any signals which include frequency components that do not belong to expected telephone signalling or data transmission schemes. However, this requires a quantity of filters which are expensive to implement in digital form.
Another known technique for detecting signalling and data signals on a telephone channel consists in analysing the average power of the signal over short time intervals and interpreting as speech any signal whose average power fluctuates, dropping below a predetermined threshold, however this technique has difficulty in distinguishing quickly between loud speech signals and signalling or data signals.